An example of a dunnage bag is described in U.S. Pat. No. 3,462,027. Generally, the prescribed practice for inflating dunnage bags and many other inflatable devices is to use a pressure gauge to check whether the required pressure has been achieved. This, however, is cumbersome procedure. Therefore a usual practice for checking the pressure in many inflatable devices is to simply press on the device and to feel how resilient it is. Obviously, this is a very unreliable method of checking the pressure of an inflated device. This is particularly true if the pressure difference with the surrounding atmosphere is in excess of 0.5 to 1.0 bar.
Dunnage bags for example typically have to be inflated to about 0.3 bar. To fill the dunnage bags, usually a compressor is used which provides a maximum pressure of about 8 bar. In practice it has been found that dunnage bags are typically inflated to pressures of up to 4.0 bar. At this pressure, a dunnage bag having a contact surface with cargo or walls of the load space of about 1 m.sup.2, which is a usual size of the contact surface, exerts a force of about 40,000 kg to the cargo. Such forces often result in damaging deformation of the cargo or of the container or transport means into which it has been loaded. Another disadvantage of overinflating dunnage bags is that it leads to a substantially increased risk of punctures and explosion. This in turns entails increased costs. Not only exploding dunnage bags, but also leaking or even empty dunnage bags form a safety hazard in that cargo is no longer prevented from shifting, which may be detrimental to the stability of the transport means and cause damage to the transport carrier and the cargo.
Also for many other inflatable devices the correct inflating pressure is critical for making full use of the properties of the device. Underinflation of an inflatable craft for example leads to a reduced stiffness of the hull while overinflation entails the risk of damage to the wall material and the seams. Underinflation of inflatable systems for fixating injured persons leads to a poor fixation while overinflation can hinder blood circulation.
In U.S. Pat. No 2,024,780 an inflatable device which includes a shut-off portion and an operating member connected to the shut-off portion for operating the shut-off portion. The shut-off portion is formed by a valve member axially displaceable in the filling passage between a position lifted from a valve seat allowing the passage of air and a position seated on the valve seat closing off the filling passage. The operating member is formed by a hollow fluid-receiving sack or diaphragm which is secured along its circumference in position within the bladder. Via a valve stem, shut-off portion is connected to a central portion of the sack or diaphragm. As the pressure in the bladder increases during inflation of the bladder, the thickness of the sack or diaphragm reduces and, via the valve stem the shut-off portion is entrained to its closing position by the movement of a central portion of the sack or diaphragm.